Casing is used when drilling wells to prevent the drill hole from caving and to preclude undesirable formation fluid movement in the surrounding rock strata. Casing is hung from a casing hanger mounted in a casing head. A "primary" seal is used in the casing hanger to seal and prevent fluid flow between the outside of the casing and the casing head.
The casing continues upwardly from the casing head and is terminated by cutting at an optimal location. The cut end of the casing is generally bevelled to remove sharp edges. Thereafter, a tubing spool is inserted over the end of the casing and attached to the casing head.
The tubing in the well acts as a conduit for the oil, gas or water of the well. It extends from the bottom of the well within the casing and upwardly a distance from the top of the cut casing. A tubing hanger is threaded on the tubing and mounted in the tubing spool. A mating bonnet is connected to the upper end of the tubing spool.
To seal the contents of the well from the primary seal between the casing and casing head, seals additional to the primary seal are used between the tubing spool and the casing. These seals are known as "secondary" seals. Seals are also used between the tubing hanger and the mating bonnet to prevent fluid flow therebetween.
Problems with existing seals have arisen for various reasons. In one secondary seal, "double-lipped" seals in a "dog bone" type shape are used. Where the end of the casing is not well bevelled or where considerable installation care is not taken, lips of the secondary seal in the tubing spool or in a bushing mounted within the tubing spool may contact the bevelled end of the casing and be damaged or jammed into the corner of the seal retaining recess and protrude into the space between the outside of the casing and the inside of the bushing. This protrusion of the lips may prevent further assembly or the lips themselves may be sheared off when the assembly is forced.
In another type of secondary seal, the seal is formed for axial pressure between the casing and the bushing only by pressurizing the outer circumferential surface with a viscous plastic fluid injection. Further, such seals cannot convey the viscous fluid through the seal to another seal situated at a radially inwardly location.